Drop charge compensated ink drop video printer

ABSTRACT

In an ink drop printer, the adverse affects of the charge on a just formed drop upon the charge on a following ink drop being formed is compensated for, enabling the use of every ink drop for printing and thus affording better and faster printing.

United States Patent [72] Inventors Robert LKeur Niles;

Vincent E. Bischofl, River Grove, both of Ill Filed Patented Assignee DROP CHARGE COMPENS ATED INK DROP [50] Field of Search 346/75, 140; 317/3; l78/6.6

Primary Examiner.loseph W. l-lartary AuorneyLindenberg, Freilich & Wasserman is Figs ABSTRACT: In an ink drop printer, the adverse affects of the charge on a just formed drop upon the charge on a following U.S. Cl...; 346/75, ink drop being formed is compensated for, enabling the use of 17/ every ink drop for printing and thus affording better and faster 1m. Cl G01d 15/18 printing.

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2.2 l4 I r v \4 I 5 =5 lRANSDUCER 0 o 0 o 0 WASTE C ATCH ER DROP CHARGE COMPENSATED INK DROP VIDEO PRINTER BACKGROUND OF THE INVENTION The present invention relates to ink drop printing apparatus and more particularly to an improved arrangement for charging ink drops.

lnk drop printing apparatus is known wherein an ink jet is emitted from a nozzle in such a manner that the ink jet stream, a short way from the nozzle tip, breaks down into ink drops. A charging ring or tunnel is placed so that the ink jet stream passes therethrough just at the point at which the ink drops begin to separate from the ink jet stream. A video signal is applied between the ring or tunnel and the nozzle whereby each ink drop which breaks from the stream assumes the charge of the video signal at the instant at which it separates from the ink stream. Beyond the charging plates or tunnel the ink drops pass through a constant electrostatic field, as a result of which they are, deviated from their straight line trajectory by an amount determined by the amplitude of the charge upon them. Adjacent to the fixed electrostatic field is a sheet of paper. Either the sheet of paper is moved or the ink drophead just described is moved so that the combination of the deflection of the ink drops and the physical motion of the paper or head can produce intelligible printing, for example, an alpha and digital characters, facsimile representations and oscillographic recording.

One of the problems which arises is that a drop which is charged and which has just broken from the ink jet stream has an electric field which operates on the drop being charged in a manner to depress or lower the charge which that drop will receive from the video signal. Similar but lesser degree effects are caused by the drops which preceded the one just broken from the ink jet stream. As a result, the following ink drop is not deflected in the electrostatic field as much as it should be. This results in printing which is not as good as it should be or which is imprecise with respect to character formation. This is especially the case where the charge on the drop which has already been charged is quite large when compared to the charge being applied to the succeeding drop. To reduce this phenomena increased drop spacing has been tried, as well as using drops with no charge for spacing charged drops. While this technique is efi'ective it slows up the printing process and reduces but does not completely eliminate the problem.

OBJECTS AND SUMMARY OF THE INVENTION It is an object of the present invention to provide a system for determining the fact that a just-formed drop has already been charged and compensating the charge being applied to a drop being formed for the adverse effect of the just-formed charged drop.

Yet another object of the present invention is an arrangement for improving the printing quality and speed of an ink drop printer.

Still another object of the present invention is the provision of a novel drop charge compensator for an ink drop printing system.

These and other objects of the present invention are achieved in an arrangement wherein means are provided for determining when a video signal has been applied to a just fonned drop, or when a just formed drop has just been charged. In that event, the video signal being applied to the succeeding drop being formed is increased in an amount to compensate for the effect of the charge on the already formed BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram illustrating the overall ink drop printing arrangement.

FIG. 2 is a representation in cross section of the ink jet stream as it is emitted from the nozzle and begins to break down into drops, shown for the purpose of illustrating the problem, which is solved by this invention.

FIG. 3 are some typical ink drop formed characters showing the adverse effect of failure to compensate for the effects of the charged ink drop on the succeeding ink drop charge.

FIG. 4 is a circuit diagram illustrating a simplified circuit arrangement for modifying existing printing systems.

FIG. 5 is a circuit diagram illustrating a charge compensating system with a direct video output.

FIG. 6 illustrates another simplified circuit arrangement for modifying existing printing systems which compensates only selected drop charges.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1

FIG. 1 shows a schematic arrangement of a known typeof ink jet printer, which includes however, a schematic representation of the circuitry required, in accordance with this invention. The ink jet printer basically includes an ink reservoir 10, which holds ink under pressure. A nozzle 12 has the ink from the ink reservoir discharged therethrough in the form of a stream 14, which is directed at the writing paper 16. An electromechanical transducer 18 squeezes the nozzle 12 at a frequency determined by the output of a sync signal generator 20. This periodic nozzle squeezing causes periodic narrowing or necking of the ink jet 14 so that drops 14' are formed along the path of the ink jet stream on its way to the paper 16. These drops are substantially uniformly sized and regularly spaced.

A small ring 22 is positioned at the location along the ink jet 14 where it begins to break into drops. The drops are formed within the ring and pass out of it through an electrical field which is established by two spaced. plates respectively 24A, 243. A bias is applied to these plates from a field bias source 26. Drops which are not used in the process of writing on the paper 16 are caught by a waste catcher 28. These drops are the uncharged drops, the charged drops being deflected between the plates 24A, 248 in a manner so that they avoid the waste catcher and fall upon the paper 16 at a location determined by the particular charge thereon.

Information for charging the drops, which determines what characters are printed on the paper 16, is provided by video signals from a data source 30. These signals are applied to a character signal function generator 32 at a rate determined by the output of the sync signal source 20. The character signal function generator 32 converts signals from the data source into a series of electrical signals which represent the voltages to be successively applied to charge successive drops. These are applied to a video processor 33 which converts the signals to a form suitable for charging the drops so they will be deposited in the desired pattern on paper. The output of the video processor 33 is applied to a video amplifier 34. The output of the video amplifier is applied between the nozzle 12 and the ring 22 whereby a charge may be applied to each drop which is formed as determined by the signals applied to the video amplifier. The amplitude of the charge is determined by the amplitude of the signal applied by the video amplifier at the time. The drop thereafter enters the field established between the plates 24A, 24B and is deflected accordingly.

The character being printed is usually constructed by deflecting the drops vertically and moving the paper horizontally, or by moving the entire ink jet printing carriage horizontally, the paper being stationary, and the drops being deflected vertically as the ink jet printing assembly is moved. In either event, the signals out of the character signal function generator comprise a sequence of voltages whose amplitudes are determined by the character signal desired to be printed.

FIGURE 2 FIG. 2 is an illustration in cross section of the end of the When a drop is charged, as represented by the plus sign in the drop 14'an electric field is established. The charging ring 22 is attempting to charge up the next succeeding drop 14"which is being formed at the end of the in jet within the charging ring 22. The efiect of the electric field caused by the charge on drop 14'on the charge being applied to the forming drop 14" is to depress the amplitude of the charge which it can assume. The amount by which the charge is depressed is determined by 4 the size of the charge ok the drop 14. The result, occurring,

by reason of the forming drop 14" having a depressed charge or a charge lower than it should have, is that it will not be deflected to'its proper location on the paper 16 whereby there may be some distortion in the character which is written thereon. A lesser effect of similar nature may occur on forming drop 14", due the the charges on previously formed drops 14' (not shown).

FIGURE 3 An illustration of the foregoing may be seen from FIG. 3 wherein two letter Ls are shown. The first of these 36 is formed with a sequence of drops which have had the proper charge applied thereto. The second of these 38 represents the appearance of the letter L which one can obtain if care is not taken to compensate for the effect of an already charged drop on a subsequent drop being formed and charged.

Since, the charge applied to the drops is unipolar, in order to compensate for the effect of a charged drop on a preceding drop being charged, all that is necessary is to change the mag nitude of the electric field being applied to the drop being formed. One manner of doing this is to increase the amplitude of the charging voltage being applied to the drop being formed. Another manner of doing this is to apply compensation to separate plates and algebraically add electric fields at a drop charging area. Still a further possibility is to inject a compensating signal at the nozzle and thereby compensate the electric field applied to the drop. However, in any of these compensating arrangements, it is first necessary to both determine whether or not a charge has been applied to an already formed drop and whether a drop to be formed is going to be used, since other wise a compensating charge may be applied to a drop being formed when it is not needed.

Referring back to FIG. 1, in accordance with one embodiment of this invention a video analyzer 40 receives the output of the video processor 33 and from it detects when a charge was applied to a just formed drop. Output from the video analyzer enables a video compensation generator circuit 42 to increment the charge being applied to the drop being formed, within the charging tunnel 22.

FIGURE 4 FIG. 4 is a schematic diagram of a simplified arrangement for compensating for the effects of a charged drop. The output of the video processor is applied both to an inverter 50 and also the K input of a memory flip-flop 52. The sync generator 20, applies clock inputs to the memory flip-flop clock input. The output of the inverter 50 is applied to the J input of the memory flip-flop 52. The Ooutput of the memory flip-flop is applied through a base resistor 56 to the base of a transistor 58. The emitter of the transistor is connected to ground. The collector of the transistor 58 is connected through a resistor 62 and a pot 60, to a feedback network 34F of the video amplifier 34 in such a way that the transistor 58 operates as a gain switch. The potentiometer 60 is set so that when the transistor 58 is conductive, the gain of the amplifier 34 is increased so as to provide the desired compensation to the drop being formed.

In operation, the presence of the clock signal from the sync generator and a video signal from the video processor 33,

memory flip-flop 52 is driven to its reset state with itsOoutput high, at which time it renders transistor 58 conductive. Upon the occurrence of the next clock pulse, if no video is being received from the output of the video generator, the output of the inverter 50 serves to set the flip-flop 52 whereby the transistor 58 is rendered nonconductive. If video is received with the next clock pulse then transistor 58 is maintained conductive.

FIGURE 5 For extremely accurate printing, it may be desirable to vary the degree of compensation being applied to the charging video signal in accordance with the amplitude of the charge which is to be applied to a drop. The reason for this is that the effects of the field established by the charge on the drop do not vary in a linear fashion with a linear increase in the drop charge amplitude. FIG. 5 shows an arrangement for establishing the degree of compensation in accordance with the magnitude of the drop charge. Only five proposed drop charge levels are represented, since these will adequately convey the technique involved. This should not be construed as a limitation upon the invention, since the principles demonstrated are applicable to systems using more or less drop charge levels than five. The output of FIG. 5 is directly applied to the video amplifier 34.

The five voltage levels Y1 through Y5 are established by the character signal function generator in response to input data signals, and correspond to drop deflections. These data signals are applied to logic circuits which select, in sequence these levels so that the ink drop printer to which these voltage levels are applied will print the character represented by the input data signals.

FIG. 5 includes an arrangement for detecting whether or not a drop has been charged. If not then the succeeding drop charge signal is derived from the digital to analog circuit on the left side of the drawing. If a drop has been charged then the succeeding drop charge voltage is derived from the digital to analog circuit on the right side of the drawing. The multiple amplitude voltage outputs from the character signal function generator 32 which are selected by the signals from the data source 34, instead of being applied to the video processor, are separately taken out. Thes outp its are equal amplitude digital signals designated as Y1 to Y5. These outputs may be derived from a counter, for example. Each one of them is applied to an inverter respectively 71, 72, 73, 74, 75. The inverter outputs consisting of the signals Y1 through Y5 are respectively applied to NAND-gates 81, 82, 83, 84, and 85.

The output of the NAND-gates 81 through 85 are each respectively applied through the respective resistors 91, 92, 93, 94, and 95, to the respective bases of transistors 101, 102, 103, 104, and 105. The emitters of each one of these transistors are connected together and to a source of biasing potential. The collectors of all of these transistors are connected through the respective resistors 1 11, 112, 113, 114 and 115 to a common bus, 116. The values of the resistors 111 through 115 increase in a manner so that the fltput volta g e on the bus 116, in response to energization by Y1 through Y5 signals, constitutes five voltages which increase in amplitude linearly. The latter five voltages are the uncompensated analog video signal. A bus 117 is provided to apply an enabling signal to NAND-gates 81, 82, 83, 84 and 85. The purpose of the enabling signal is to turn on the matrix described above.

The bus 116 is connected to the base of a transistor 118. The transistor collector is connected to a source of operating potential. The transistor emitter is connected to a fixed resistor 122, and latter is connected to a negative operating potential source. A potentiometer is connected between the base of the the transistor 118 and ground and is used to adjust the overall value of all signals summed at this point. The output applied to the video amplifier, or the video output, is derived from the emitter of transistor 118.

-A second matrix, substantially Similar to the one just described is employed to generate. a compensated video signalQ-The second matrix includes NAND-gates 131, 132, 133, 134, and 135. A second input to these NAND gates is the output of an inverter .136. Each one of these NAND gates has its output respectively connected through the respective resistors 141, 142, 143, 144, and 145 to the base of respective transistors, 151, 152 153, 154, and ISSQThe emitters of these transistors are all connected together to a source of operating potential-.The collectors of each one of these transistors is connected through potentiometers respectively 161, 162, 163, 164 and 165, to the common bus 116. These potentiometers are adjusted to produce the non linearly interrelated voltage values to provide a compensated video signal. The bus 166 also connects to base of the video amplifier transistor 118. It will be shown subsequently that a bias or pedestal voltage is also added to the voltage on bus 116. This bias or pedestal voltage voltage establishes a minimum drop charging voltage for a drop to be deflected above the waste catcher 28 and impinge upon the recording medium. A bus 167 is provided to perform the same function for the second matrix that was performed by bus 117 for the first matrix. That is, an enabling signal is fed to the second matrix to turn it on.

The control circuits for the digital to analog converters just described include a flip-flop 168 having a J input and a K input. This flip-flop stores the fact of the occurrence of the charge on the last drop. Video output from the character generator 32 is applied to the J input of the flip-flop and through an inverter 170, to the B input of the flip-flop. A clock signal is applied to the clock terminal of the input of the flipflop. Accordingly, in the presence of video the flipflop is driven to its set state upon the occurrence of the clock signal. In the absence of video it is driven into its reset state.

The Q tput of the flip-flop 168 is applied to a NAND-gate 172. The output of the flip-flop 168 is applied to a NAND- gate 174. A second input to the two NAND gates is the clock signal. A third input to the two NAND gates is an incoming video signal. This is a signal from the character signal function generator 32 which provides an output designated as W65 when it generates a video signal to be applied to charge the drop which is forming. This video signal is applied to an inverter 176, the output of which is applied to both NAND-gates 172 and 174, as previously described.

From the foregoing it will be seen that the flip-flop 168 stores the fact of the application of a charge to a drop which has been formed. The fact that a charge will be applied to the succeeding drop or the one presently being formed is determined by the video signal which is applied through the inverter to both NAND gates. NAND-gate 172 is enabled if the previous drop was charged. Its output is applied to the inverter 136, and also to an AND-gate 180. The output of the inverter I36 enables NAND-gates 131 through 135 so that the charging signal for the drop being formed will be one which includes compensation for. the effects of the charging signal applied to the drop preceding it.

If a drop, which has just been formed, is not charged, then flip-flop 168 is reset upon the occurrence of the next clock pulse. NAND-gate 174 provides an output signal in the presence of the next video signal thereby applying an output to an inverter 187 coupled to bus 117. This enables NAND- gates 81 through 85. As a result, an output is applied to the bus 116. This output, which is the uncompensated video signal, is applied to the base of the video amplifier transistor 1 18.

A, fixed bias or pedestal signal is also added to the video signal on bus 116. The output of the NAND-gates 172 and 174 are applied to a NOR-gate 180. In the presence of an output from either NAND-gate 172 or 174, NOR-gate 180 applies an output to an inverter 184, the output of which is applied to the base of a transistor 186. The emitter of this transistor is connected to an operating potential source. The collector is connected through a potentiometer 188 to the bus 116. Potentiometer 188 is adjusted so that the desired bias or pedestal signal is provided.

is used when the preceding drop does have a charge. The out-- put voltages of both converters have added thereto a bias or pedestal video signal, that signal assures that the drops are directed to the recording medium.

FIGURE 6 In the event that it is desired to apply compensation for only certain levels of charge, then a video analyzer circuit 190, such as shown in FIG. 6, may be employed. Here, flip-flop 191 is'set in the presence of video and a clock signal and is reset by the clock in the absence of input video. This is accomplished by applying the input digital video signal to both the J input and through an inverter 192 to the K input of the flip-flop 191. Two NAND gates respectively 194 and 196 have a first input the Q output of the flip-flop and as a second input the clock signal. The third required inputs to these NAND gates is? Y comprises of the outputs of the character signal function generator, Y1 through Y5, for which compensation is desired. The third required input to NAND-gate 194 is comprised of a Y1+Y5 signal. The third required input to NAND-gate 194 is comprised of a Y3Y4 signal.

The output of NAND-gate 194 is connected through a transistorized switch 198 and an adjustable potentiometer 199 to a summing junction 200. The output of the NAND-gate 196 is connected through a transistorized switch 201 to a potentiometer 202 and then to the summing junction 200. The summing junction is connected to the video processor 33 input, instead of to the video amplifier. The potentiometers 199 and 202 are set to provide the desired compensating signal levels.

There has accordingly been described hereinabove a novel and useful arrangement which minimizes the effects of the charge, on a drop in the process of being formed and charged, by a drop that has already been formed and charged. As a result, every drop can be used in the ink drop writing process. This facilitates formation of, for example alpha and digital characters, facsimile printing and oscillographic recording. Further, the appearance of the character formation and printing is considerably improved.

I claim: 1. In an ink jet printing system wherein a stream of ink drops are directed at a recording medium, wherein video signals derived from a source are used to establish an electric field to apply charges to successive ones of said drops in their path toward said recording medium whereby said drops may be deflected in accordance with the amplitude of said video signals to enable intelligible information to be recorded, the improvement comprising:

means for determining that a charge responsive to a video signal has been applied to a just formed ink drop and providing an output indicative thereof,

means responsive to said output for establishing an electric field for charging a succeeding ink drop with a charge having a magnitude for minimizing the effects on said succeeding ink drop charge of the charges on the previously formed ink drops.

2. Apparatus as recited in claim 1 wherein said means for changing the electric field for charging a succeeding drop includes digital to analog means for providing output signals to establish said electric field and to charge succeeding ink drops, said signals being nonlinearly interrelated in amplitude.

3. Apparatus as recited in claim 1 wherein said means responsive to said output for charging a succeeding ink drop includes video amplifier means for amplifying video signals received from said source,

means driven responsive to said video amplifier means output for applying a charge to a succeeding ink drop, and

control amplifier means responsive to the output of said means for determining for increasing the gain of said video amplifier means.

4. Apparatus as recited in claim 1 wherein said means for charging a drop includes means for detecting which of the video signals will result in the application of predetermined charge amplitudes to said drops and producing an output invideo signals will result in the application of predetermined charge amplitudes to said drops and producing an output indicative thereof,

means responsive to said video signals to produce a charge signal representative of the amplitude of the charge to be applied to the succeeding ink drop,

' gate means for selecting predetermined ones of said charge signal andproducing an output for compensating for the effect of charges on previously formed ink drops,

means for generating a drop charge signal having an amplitude determined by each video signal, and means for charging a drop with said combined signal.

6. In an ink jet printing system wherein a stream of ink drops are directed at a recording medium, wherein video signals derived from a source are used to apply charges to successive ones of said drops in their path toward said recording medium whereby said drops may be deflected in accordance with the amplitude of said video signals to enable intelligible information to be recorded, the improvement comprising:

means for determining that a charge responsive to a video signal has been applied to a just formed ink drop and producing an output signal representative thereof,

a first means for generating a plurality of separate, increasing amplitude, drop charging voltages which are substantially linearly interrelated to one another,

a second means for generating a plurality of separate, in-

creasing, amplitude, drop charging voltages which are non linearly related to one another to compensate for the effects of the charge on an already charged drop,

means for selecting which of the drop charging voltages of said first or said second means is to be used for drop charging, and

means responsive to the presence of an output signal for enabling said second means and to the absence of an output signal for enabling said first means.

7. In an ink jet printing system wherein a stream of ink drops are directed at a recording medium, wherein video signals derived from a source are used to apply charges to successive ones of said drops in their path toward said recording medium whereby said drops may be deflected in accordance with the amplitude of said video signals to enable intelligible information to be recorded, the improvement comprising:

means for detecting whether or not a charge responsive to a video signal has been applied to a just formed drop and producing a first signal indicative of the application of said charge and a second signal indicative of the fact that a charge has not been applied to said just formed drop,

means responsive to said first signal for generating a third signal having an amplitude for compensating for the effect of the charge on said just formed drop on the charge to be applied to a drop being formed,

means for adding said compensating signal to the video signal to be applied to said drop being formed to produce a combined signal means for charging said drip being formed with said combined signals, and

means responsive to said second signal to apply the succeeding video signal to the drop being formed.

8. In an ink jet printing system wherein a stream of ink drops are directed at a recording medium, wherein video signals derived from a source are used to apply charges to successive ones of said drops in their path toward said recording medium whereby said drops may be deflected in accordance with the amplitude of said video signals to enable intelligible informa tion to be recorded, the improvement comprising:

means for detecting whether or not acharge responsive to a video signal has been applied to a just fonned drop and producing a first signal indicative of the application of said charge and a second signal indicative of the fact that a charge has not been applied to said just formed drop,

means responsive to said first signal for generating a third signal having an amplitude for compensatingfor the effect of the charge on said just formed drop on the charge to be applied to a drop being formed,

means for charging said drop being formed,

means for applying said third signal to said means for charging said drop being formed,

means responsive to said second signal to generate a fourth signal having an amplitude which does not compensate said drop being formed for a charge on a just formed drop, and

means for applying said fourth signal to said means for charging a drop being formed.

9. In an ink jet printing system of the type wherein a charge is applied to a drop of ink having an amplitude determined by the deflection desired for said drop, means for compensating for the effects of the charge on a just formed drop on the charge being applied to a drop being formed comprising:

first digital to analog converter means for generating successive linearly increasing first output signals,

second digital to analog converter means for generating successively increasing nonlinearly related second output signals,

detecting means for producing a third signal indicative of the fact that the charge had been applied to an ink drop and for producing a fourth signal for indicating thata charge had not been applied to an ink drop,

a source of digital signals representative of signals desired to be applied to charge said ink drops, means responsive to signals from said source of digital signals and a third signal for enabling said second digital to analog converter to produce an output second signal,

means responsive to said digital signals and a fourth signal for enabling said first digital to analog converter to produce an output first signal, and

means for charging said drop being formed with either the output of said first or second digital-to analog converter.

10. Apparatus as recited in claim 9 wherein each of said digital to analog converter means includes a plurality of separate NAND gates one for each digital signal to be converted to an analog signal, each NAND gate including as a first input the digital signal and as a second input an output from said detecting means,

a transistor for each NAND gate, each base collector and emitter electrodes, means connecting the output of each NAND gate to its associated transistor base,

means for applying operating potential to the emitters of all said transistors, resistance means for each transistor collector connecting each transistor collector to a common junction, and means for charging ink drops being formed with a voltage established at said common junction.

lll. Apparatus as recited in claim 10 wherein said resistance means for said first digital to analog converter means have increasingly greater resistance values which are linearly interrelated.

12. Apparatus as recited in claim 11 wherein said resistance means for said second digital to analog converter means have increasingly greater resistance values which are nonlinearly interrelated.

transistor having 

1. In an ink jet printing system wherein a stream of ink drops are directed at a recording medium, wherein video signals derived from a source are used to establish an electric field to apply charges to successive one''s of said drops in their path toward said recording medium whereby said drops may be deflected in accordance with the amplitude of said video signals to enable intelligible information to be recorded, the improvemeNt comprising: means for determining that a charge responsive to a video signal has been applied to a just formed ink drop and providing an output indicative thereof, means responsive to said output for establishing an electric field for charging a succeeding ink drop with a charge having a magnitude for minimizing the effects on said succeeding ink drop charge of the charges on the previously formed ink drops.
 2. Apparatus as recited in claim 1 wherein said means for changing the electric field for charging a succeeding drop includes digital to analog means for providing output signals to establish said electric field and to charge succeeding ink drops, said signals being nonlinearly interrelated in amplitude.
 3. Apparatus as recited in claim 1 wherein said means responsive to said output for charging a succeeding ink drop includes video amplifier means for amplifying video signals received from said source, means driven responsive to said video amplifier means output for applying a charge to a succeeding ink drop, and control amplifier means responsive to the output of said means for determining for increasing the gain of said video amplifier means.
 4. Apparatus as recited in claim 1 wherein said means for charging a drop includes means for detecting which of the video signals will result in the application of predetermined charge amplitudes to said drops and producing an output indicative thereof, and means responsive to each of said outputs for generating a charging signal having an amplitude to compensate for the effects of charges on previously formed ink drops.
 5. Apparatus as recited in claim 1 wherein said means for charging a drop includes means for detecting which of the video signals will result in the application of predetermined charge amplitudes to said drops and producing an output indicative thereof, means responsive to said video signals to produce a charge signal representative of the amplitude of the charge to be applied to the succeeding ink drop, gate means for selecting predetermined ones of said charge signals and producing an output for compensating for the effect of charges on previously formed ink drops, means for generating a drop charge signal having an amplitude determined by each video signal, and means for charging a drop with said combined signal.
 6. In an ink jet printing system wherein a stream of ink drops are directed at a recording medium, wherein video signals derived from a source are used to apply charges to successive one''s of said drops in their path toward said recording medium whereby said drops may be deflected in accordance with the amplitude of said video signals to enable intelligible information to be recorded, the improvement comprising: means for determining that a charge responsive to a video signal has been applied to a just formed ink drop and producing an output signal representative thereof, a first means for generating a plurality of separate, increasing amplitude, drop charging voltages which are substantially linearly interrelated to one another, a second means for generating a plurality of separate, increasing, amplitude, drop charging voltages which are non linearly related to one another to compensate for the effects of the charge on an already charged drop, means for selecting which of the drop charging voltages of said first or said second means is to be used for drop charging, and means responsive to the presence of an output signal for enabling said second means and to the absence of an output signal for enabling said first means.
 7. In an ink jet printing system wherein a stream of ink drops are directed at a recording medium, wherein video signals derived from a source are used to apply charges to successive one''s of said drops in their path toward said recording medium whereby said drops may be deflected in accordance with the amplitude of said video signals to enable intelligible information to be recorded, The improvement comprising: means for detecting whether or not a charge responsive to a video signal has been applied to a just formed drop and producing a first signal indicative of the application of said charge and a second signal indicative of the fact that a charge has not been applied to said just formed drop, means responsive to said first signal for generating a third signal having an amplitude for compensating for the effect of the charge on said just formed drop on the charge to be applied to a drop being formed, means for adding said compensating signal to the video signal to be applied to said drop being formed to produce a combined signal, means for charging said drip being formed with said combined signals, and means responsive to said second signal to apply the succeeding video signal to the drop being formed.
 8. In an ink jet printing system wherein a stream of ink drops are directed at a recording medium, wherein video signals derived from a source are used to apply charges to successive one''s of said drops in their path toward said recording medium whereby said drops may be deflected in accordance with the amplitude of said video signals to enable intelligible information to be recorded, the improvement comprising: means for detecting whether or not a charge responsive to a video signal has been applied to a just formed drop and producing a first signal indicative of the application of said charge and a second signal indicative of the fact that a charge has not been applied to said just formed drop, means responsive to said first signal for generating a third signal having an amplitude for compensating for the effect of the charge on said just formed drop on the charge to be applied to a drop being formed, means for charging said drop being formed, means for applying said third signal to said means for charging said drop being formed, means responsive to said second signal to generate a fourth signal having an amplitude which does not compensate said drop being formed for a charge on a just formed drop, and means for applying said fourth signal to said means for charging a drop being formed.
 9. In an ink jet printing system of the type wherein a charge is applied to a drop of ink having an amplitude determined by the deflection desired for said drop, means for compensating for the effects of the charge on a just formed drop on the charge being applied to a drop being formed comprising: first digital to analog converter means for generating successive linearly increasing first output signals, second digital to analog converter means for generating successively increasing nonlinearly related second output signals, detecting means for producing a third signal indicative of the fact that a charge had been applied to an ink drop and for producing a fourth signal for indicating that a charge had not been applied to an ink drop, a source of digital signals representative of signals desired to be applied to charge said ink drops, means responsive to signals from said source of digital signals and a third signal for enabling said second digital to analog converter to produce an output second signal, means responsive to said digital signals and a fourth signal for enabling said first digital to analog converter to produce an output first signal, and means for charging said drop being formed with either the output of said first or second digital to analog converter.
 10. Apparatus as recited in claim 9 wherein each of said digital to analog converter means includes a plurality of separate NAND gates one for each digital signal to be converted to an analog signal, each NAND gate including as a first input the digital signal and as a second input an output from said detecting means, a transistor for each NAND gate, each transistor having base collector and emitter electrodes, means connecting the output of each NAND gate to its associated transistor basE, means for applying operating potential to the emitters of all said transistors, resistance means for each transistor collector connecting each transistor collector to a common junction, and means for charging ink drops being formed with a voltage established at said common junction.
 11. Apparatus as recited in claim 10 wherein said resistance means for said first digital to analog converter means have increasingly greater resistance values which are linearly interrelated.
 12. Apparatus as recited in claim 11 wherein said resistance means for said second digital to analog converter means have increasingly greater resistance values which are nonlinearly interrelated. 